The present invention relates to gas chromatography systems for generally continuously sampling chemical samples and detecting desired compounds therefrom. In particular, the subject invention is directed to gas chromatographic (GC) column modules for temperature programmed analysis as well as a gas chromatography oven having a door (or a wall) designed specifically to be removably integrated with the column GC modules external the oven cavity in order that the GC oven may be operated isothermically at an elevated temperature for efficient gas chromatography analysis.
Moreover, the present invention relates to a gas chromatography system which includes capillary gas chromatographic column members, temperature sensing mechanisms and heating mechanisms formed into gas chromatographic column modules which are located at the door of a gas chromatography oven in a manner to optimize thermal effect and produce an overall low power consumption system.
In the field of gas chromatography, sample tests are typically administered in a temperature controlled oven. For example, as described in U.S. Pat. Nos. 4,420,679; 5,665,314; and, 5,830,353, a capillary GC column which is usually contoured as an extended tube wound in a generally circular fashion is suspended in the GC oven between a sample injector and a sample detector.
The temperature programming of capillary gas chromatography columns is standardly practiced by electronic control of the temperature of a GC oven containing the gas chromatography column. To achieve rapid and uniform temperature response of the gas chromatography column assembly to temperature changes in the oven, capillary gas chromatography columns are standardly packaged by winding the columns on a wire frame support. The winding of the columns on the wire frame support provide extensive surface contact of the capillary gas chromatography column with the heated air in the oven for rapid temperature calibration of the capillary gas chromatography column with the oven air. In laboratory gas chromatography ovens, the air within the oven is typically mixed with a fan to achieve temperature uniformity within the oven.
When a sample is injected through the injector port, it travels through the column until it reaches the detector port. For the standard practice of temperature programming in gas chromatographic analysis, the temperature in the oven containing the gas chromatography column is gradually increased to extend the range of gas chromatography separation capability. The use of capillary columns have become standard practice in laboratory gas chromatography instrumentation and the wide range of separation capability has been made possible through variation of the chemical compositions of the polymers which coat the inner walls of the capillary gas chromatography column. A number of polymer coatings are commercially available for capillary gas chromatography columns having standard thickness, column length, and column inner diameters to optimize the chemical separation required of the gas chromatography.
The need for rapid temperature programming of miniature chromatographic analysis instrumentation may be accomplished by several design techniques. For example, as described in U.S. Pat. No. 5,014,541, the standard gas chromatography oven is replaced by a tubular heat conductor support on which the gas chromatography column is wound. A heating element within the tubular support is used for temperature programming. In another technique, described in U.S. Pat. No. 3,159,996 a glass tube with three parallel bores and sufficient length to contain a heater wire is provided. A resistance thermometer wire (temperature sensor), with the remaining bore coated on the inside functions as a gas chromatography column. As described in U.S. Pat. No. 5,005,399, a thin film coated capillary gas chromatography column is wound on a mandrill consisting of an insulating material. Electrical current passed through the thin film surrounding the gas chromatography column is used to resistively heat the column.
As described in U.S. Pat. Nos. 5,782,964; 6,209,386; and 6,217,829, gas chromatography systems include a capillary gas chromatography column member is provided which contains a chemical sample to be analyzed, a heating mechanism which extends through the length of the capillary gas chromatography column member, and a temperature sensing mechanism for measuring the temperature of the capillary gas chromatography column member which is mounted adjacent to the column member and are bound together into the gas chromatography system. The gas chromatography systems are placed into a gas chromatography containing the column through which chemical samples are passed and thereby separated.
The ability to readily use these commercially available gas chromatography column technologies in small portable gas chromatography instruments (modules) is desirable for the practical realization of similar analytical capabilities in portable or small gas chromatography instruments.
A number of different technologies have been developed as alternatives to standard air circulation ovens for temperature control in gas chromatography. These technologies have sought to achieve faster GC column heating rates (i.e., fast temperature programming, especially at elevated temperatures), smaller instrument sizes, reduced power consumption, etc. A straightforward approach to working with injectors and detectors in existing GC ovens is to place a heating device directly within the oven to apply local temperature control of the column between its connections to the injector and detector within the GC oven.
Using this approach, resistive heating technology for direct heating of a capillary GC column has been packaged for use in a standard GC oven commercially provided by Thermedics Detection, Inc. (Woburn, Mass.). Thermedics Detection, Inc. has developed a xe2x80x9cretrofitxe2x80x9d GC system in which a column module is placed within the GC oven, coupled between the detector and injector connection ports, and is remotely controlled by an electronics package separate from the GC instrument. Such a direct approach allows the Thermedics Detection, Inc. system to utilize the fast column heating technology in conjunction with existing sample preparation/injection technique, as well as detection hardware without any changes thereto. Further, data acquisition, analysis and management software that commercially exists may be used in such a system.
In an alternative approach, designed by MT Systems (as described in U.S. Pat. No. 6,093,921), a microwave heated capillary column is placed in a small microwave cavity within the laboratory GC oven to achieve a retrofit GC system for faster temperature programming. This microwave cavity similarly heats most of the columns spanning from the injector to the detector in the oven and is remotely powered by an electronics package separate from the GC instrument.
One of the technical challenges, however, in integrating auxiliary column heating technology into an air circulation GC oven, is the prevention of xe2x80x9ccold spotsxe2x80x9d along the GC column between the injector and detector ports. In the practice of temperature programmed GC, the increased temperature selectively passes compounds having specific boiling points through the column to effectively separate compounds having a wide range of boiling points as known to those skilled in the art. Any xe2x80x9ccold spotsxe2x80x9d in the sample path slow or effectively halt the transit of the sample vapor through the column, thus resulting in delay of the analysis along with degradation or complete loss of detection of the higher boiling point component from a sample injected for analysis.
The injectors and detectors of laboratory GC instruments generally have their own temperature control independent of the oven temperature. The injector/detector""s temperatures are typically set close to the maximum analysis temperature required to insure that the samples are vaporized quickly and propagate through the GC column without experiencing xe2x80x9ccold spotsxe2x80x9d at either the injector or detector areas.
While it is standard to set both of these components to operate isothermally at an elevated temperature, chromatographers and instrument designers have found that the GC column attachments to both the injector and detector which descend down into the oven are poorly heated when the oven is not heated. In practice, much of the heating of the injector""s and detector""s extensions into the oven are accomplished by the air circulation oven rather than the heaters built into the bodies of the devices external to the oven.
Temperatures as low as 80xc2x0 C. have been measured within the column attachment portion of detectors (extending inside the oven) whereas the portions extended into the oven near the oven wall were heated to 350xc2x0 C. using standard heaters built into the detector.
Similar to the approaches used by Thermedics Detection, Inc., and MT Systems, Applicants of the present invention perform a direct attachment of a resistance wire heated GC column assembly to the injector and detector within the GC oven in order to eliminate the problem of the xe2x80x9ccold spotsxe2x80x9d at the internal portion of the injector and detector.
In order to solve the problems associated with poorly heated detector and injector ports, the Applicants of the present invention, as well as Thermedics Detection, Inc. and MT Systems, have developed specific hardware for GC systems and added heating zones to the design. For example, the Applicants of the present invention have built supplemental heating enclosures that were insulated and could be opened in order to allow connections to be made. Prior art systems put insulation around the injector and detector ports and fed their connections up into the heated area with a less modular approach.
As shown in FIG. 1, with reference to the prior art, a GC module 10 is placed inside the oven 12. The electronic heater control 14 placed outside the oven 12 is coupled to GC module 10 for controlling the temperature thereof. The ends of the GC column, such as the GC column ends 16 and 18 extend from the GC module 10 to the injector port 20 and to the detector port 22. Supplemental heater zones 24 and 26 heated isothermally by means of electronic heater control connections 28 and 30 are arranged between the GC module 10 and the injector and detector ports, 20 and 22 respectively.
Application of heat to the regions of the detector and the injector ports internally of the oven has resolved the problem and provided chromatography results over the full range of expected boiling points. However, the direct approach of placing the GC module 10 within the oven and providing supplemental heaters which could alleviate the heating shortcomings of the detector and injector ports, have caused the following problems:
(a) Need for Disrupting the Normal Operation of the GC System.
Since the GC module 10 cools with high efficiency, it is difficult to heat this device to high temperatures in the presence of circulating cool air in an unheated air-circulation GC oven. It is therefore necessary to prevent a large fan positioned in the back of the oven (typical for standard GC ovens) from operating while the GC module is being heated. Disconnection of the fan from the GC circuits, on the other hand, can lead to overheating of the GC module due to the buildup of heat within the oven. Additionally, the circuits of many GC instruments do not operate with the fan disconnected for safety reasons. Thus both the fan and the oven heaters may require disconnection. However, non-operation of the oven results in temperature control errors as the built-in temperature sensors report a temperature control failure. This may shut down the electronics of major systems of the GC instrument.
To further defeat the GC oven""s temperature sensor, typically a platinum RTD, the sensor can be disconnected and bypassed with a fixed resistor. By using a low temperature coefficient resistor, drift may be minimized and the GC instrument may be programmed to xe2x80x9coperatexe2x80x9d isothermally at a temperature equivalent to this resistor.
However, the need to interfere with the circuitry and operation of the GC instrument is a concern as this potentially voids manufacturer""s warranties and leads to damage of the instruments if not properly accomplished. Further, this provides a significant obstacle to ease of use requiring the user to correctly dismantle the GC instrument and disable or bypass these components.
(b) The Generation of a Significant Heat Load in the GC Oven Making it Difficult to Cool.
The heating of the GC module 10, the injector body 20, the detector body 22, and especially the supplemental heaters 24, 26 of FIG. 1, together generate a large heat load within the confines of the GC oven 12. This makes it difficult to cool the device to moderate starting temperatures in the neighborhood of ambient temperatures (e.g., 30-40xc2x0 C.). Prior art systems approached this problem by temperature programming the oven over a limited range of temperature while the GC column is (e.g., 40-70xc2x0 C.) effectively absorbing the heat given off by the GC column while maintaining a temperature control of the oven. This constant cooling by the fan in the relatively cooler oven, however, may potentially interfere with the local heating of the GC column, especially as the local GC column temperatures greatly exceed the air circulation oven temperature. This requires the GC column heating device to be operated with much lower thermal efficiency and possible degradation of its ability to properly control temperatures at higher operating temperatures. Further, the oven starting temperature must be re-established before another analysis can begin.
(c) Free Column Ends for Connections are not Always Desirable.
For certain applications requiring robustness of the components, it is desirable to have the fragile column ends 16, 18 terminate in rigid connectors 32, 34 on the module 10. Without such connectors, the fragile column ends are susceptible to damage. If either of these ends are broken or trimmed to a length too short to make the required connections the integrity of the module 10 is compromised. If a rigid connector is used such as a xe2x80x9cunionxe2x80x9d designed for capillary chromatography is used, it must also be heated. Chromatography unions are typically steel and very massive compared to the column itself and are difficult to heat compared to the GC column which is integrated with fine, temperature-controlled heating wire described in previous paragraphs with regard to U.S. Pat. Nos. 6,217,829; 6,209,386; 5,782,964.
For this reason, a supplemental heating system for the injector and detector in the oven would likely need to also heat these unions further increasing their size and complexity. If free ends 16, 18 of the column are alternatively used, then utmost care is required to avoid breakage during the simultaneous insertion of the free column ends into the injector 20 and detector 30 (especially through the supplemental heating means 24, 26).
(d) Conventional Column Connections are Very Awkward With Minimum Size Supplemental Heaters.
Injectors and detectors are both designed for the column to be manually inserted oftentimes to specific distances. Free ends 16, 18, as part of the GC module 10, are difficult to work with and are likely to result in breakage. If the GC column is modular with connectors 32, 34, then shorter pieces of capillary column may be connected between the injector 20 or detector 22 and the module 10 through the supplemental heaters 24, 26. To minimize the power dissipation, these heaters must have minimum surface areas and should be insulated, while having a span from the detector or injector ports to the module. It has been found difficult to heat distances of 3 to 4 inches from each of these injector/detector ports 20, 22 to the module 10 to temperatures of 300-350xc2x0 C. without a large heat load being introduced to the oven. If these are made minimal in size, in order to reduce heat dissipation and power consumption, it becomes increasingly difficult to manipulate and thread the GC column through these heaters without breakage of the fragile capillary.
Further, the required thickness for insulation of 0.5-1.0 inch or more reduces the area available within the oven and further limits the space available for handling and threading the column connection pieces into the injector, detector and module connectors. The approach taken by prior art systems require connections that can be very awkward, delicate, and prone to difficulties with xe2x80x9ccold spotsxe2x80x9d or uneven heating.
(e) Difficult to Trim Back Column Connection to Injector.
Many GC analyses require the injection of contaminated samples which contain materials and possibly particulates which degrade the chromatography performance of the entire column. These materials typically do not migrate far down the column, but instead contaminate the coating inside the column for a short distance from the injector. Certain common analysis methods for semi-volatile compounds are prone to such contamination and the GC separation performance is typically regained by trimming back a length of the GC column and reconnection to the injector. While such a xe2x80x9ccolumn cutbackxe2x80x9d approach is possible with GC modules by replacing the column length used to connect the module to the injector, such a replacement within the confines of the insulated supplemental heater system is difficult compared to cutting and reconnecting a column within the large and easily accessible space of a standard GC oven.
(f) Different Designs Required for Every Possible Injector and Detector Geometry.
Since GC ovens often have multiple openings that may be used for injectors or detectors, many different physical placements of these components are possible. This includes detectors such as mass spectrometers which often enter the oven from a side of the oven. The need to make supplemental heated zones which can accommodate GC column modules then requires many different geometries or imposes difficult flexibility requirements on the mechanical designs. This can result in an impractical number of models or design variations.
Thus, it would be highly desirable in the field of chromatography to have a gas chromatography system free of the aforesaid shortcomings of designs of the prior art.
It is therefore an object of the present invention to provide a gas chromatography system employing gas chromatography column combined with heating elements in a single portable module which is replaceably integratable with a door of the gas chromatography oven.
It is a further object of the present invention to change a design of the conventional gas chromatography oven by replacing a door (or any wall thereof) with a novel door (or a wall) which is provided with a module receiving slot in which the column module may be easily secured to extend external the oven and wherein free column ends projecting from the gas chromatography column module to the injector and detector port inside the oven are heated isothermally.
It is another object of the present invention to change the fabrication of existing gas chromatography systems by replacing a conventional door with a novel door construction which allows it to be integrated with the column module with no need to interfere with a fan, heater, or the temperature sensors, and with no risk of misconnections or damaging the electronics of the existing GC system.
It is a still further object of the present invention to provide a gas chromatography system where the heat generated by the modules is exterior to the oven, while the oven is heated isothermally to heat the module connections and extensions of the detector and injector into the oven. Thus, not requiring cool down of the oven for each analysis, since the module contains most of the GC column length outside the oven which is rapidly heated and cooled.
A further object of the present invention is to provide a gas chromatography system where it is possible to combine in a single system both fast temperature cycles applied to the low thermal mass component of the module outside of the oven and heating of the larger thermal mass components within the oven while maintaining an interface between the components inside the oven and outside the oven in which the oven heat does not significantly externally conduct and interfere with the temperature cycling of the module components.
It is another object of the present invention to provide a simple replacement of column modules as well as easy access to the injector and detector ports for connection thereof to the capillary connectors of the module.
A further object of the present invention is to provide a gas chromatography system permitting the inclusion of multiple GC column modules, thereby providing the opportunity for more advanced GC analysis on two (or more) different GC columns and then detecting the results with a pair (or more) of detectors. In this manner, multi-column analyses may be accomplished where not only multiple temperature programs can be conducted but the plurality of modules may perform sophisticated temperature programs. For example, the exterior air cooled modules may perform multi-segment temperature programs which contain negative ramps, as well as positive ramps for advanced applications of gas chromatography analysis. The independent temperature programming of multiple modules also allows the sequential operation of columns in series or in combination with valves and individual detectors.
It is still another object of the present invention to provide a column module for the gas chromatography system which includes a column module portion combinable with a transfer line module portion which may be inserted into the module receiving openings provided in the oven door either simultaneously or in a predetermined order.
According to the teachings of the present invention, a gas chromatography (GC) system includes a GC oven having an oven cavity enveloped by walls and a door wherein a door has up to four module receiving openings defined therein, and wherein a wall of the oven has openings for injector and detector ports. Up to four GC column modules may be removably secured within the module receiving openings of the oven door for gas chromatography analysis.
Each GC column module comprises:
a module housing formed of a perforated stainless steel, a capillary column where the main coil is contained within the module housing and free ends extend beyond the walls of the module housing,
a pair of transfer lines each coupled to a respective free end of the main column,
a heater wire positioned adjacent the transfer lines,
a pair of chromatography connectors with each coupled to a respective one of the transfer lines and extending from the GC column module into the oven cavity, and
a mechanism for securing the column modules to the oven door.
The GC system further includes first and second GC column lengths positioned within the oven cavity. The first GC column length extends between a respective gas chromatography connector and the injector port, and the second GC column length extends between another gas chromatography connector on the module and the detector port.
The system further includes a temperature control unit attached to the oven door outside the oven cavity operatively coupled to the heater wire and the transfer line. A heater is positioned within the oven cavity for heating the same isothermally.
The subject oven door of a new design replaces the conventional oven door by means of hinges positioned at one side edge of the door and a latch mechanism positioned at another side edge of the oven door for hermetically closing the oven.
The oven door constitutes a multi-layer structure providing for a sufficient thermal isolation between the oven cavity and the external surroundings. Particularly, the oven door includes an inner plate having feed through holes for projection of the module""s chromatography connectors therethrough, a layer of insulating material attached to the inner plate, an insulation retaining plate attached to the layer of insulation material, an aluminum insulation frame attached to the insulation retaining plate, and an outer door attached to the aluminum insulation frame.
Each of the layers of the oven door has slots defined therein in aligned disposition, thus defining the module receiving openings of the oven door for receiving the face end of the module therein.
Preferably, the temperature control unit includes heating control circuits, electrical connections to the GC modules, a microprocessor and a user interface.
Positioned in close proximity to each module is a cooling fan attached to the oven door externally the oven cavity.
The GC column module is envisioned in two alternative designs. One design includes:
a module base plate,
a module cover attached to the module base plate, thus forming a module housing,
a capillary column in the module housing,
a pair of transfer lines,
a module face plate attached to the module housing at one end through a face plate insulation (the face plate insulation and the face plate both have openings for projecting the free ends of the capillary column therethrough), and
a pair of module clamps attached to the module face plate in alignment with the openings.
Each module clamp has an aperture provided for projection therethrough of a respective one of the chromatography columns.
Each transfer line is a thin walled tube formed of steel or like material heated by resistance wire and sleeving the free ends of the main capillary column. Each transfer line is heat compensated by reducing the number of heater wire windings around the transfer line along the length of the transfer lines entering into the oven cavity.
In an alternative embodiment, each GC column module includes a column module having the module housing containing the main coil. A transfer line module is included which comprises a transfer line housing, a pair of tubes secured to and extending through the transfer line housing for sleeving the free ends of the capillary column, a pair of bars extending from the transfer line housing at a face end thereof in proximity to the pair of tubes, and a mechanism for securing the transfer line module to the column module. In order to integrate the GC module to the oven door, the module of this embodiment may be either assembled before inserting the module into the modular receiving opening in the oven door or the transfer line module may be secured to the module receiving opening and the column module can be further coupled to the transfer line module.
These and other features and advantages of the subject invention will be more fully understood from the following detailed description of the accompanying Drawings.